1. Field of the Invention
This invention relates to a method for determining the porosity of earth formations penetrated by a borehole. More specifically, the invention relates to a method whereby inaccuracies in the determination of the porosity as a result of standoff from the borehole wall may be substantially eliminated.
2. Description of the Related Art
In hydrocarbon exploration and production, it is of prime importance to determine (a) if a given earth formation contains hydrocarbon, (b) the amount of hydrocarbon within the formation, and (c) the producibility of the hydrocarbon in place within the formation. The amount of hydrocarbon present within a formation is a function of the pore space or the “porosity” of the formation. In drilling wells for the production of hydrocarbon, and even after those wells have been drilled, it is desirable to measure the porosity of each prospective hydrocarbon producing formation penetrated by the borehole. It is even more desirable, for economic and operational reasons well known in the art, to determine the porosity of prospective formations during the actual drilling of the borehole.
Porosity measurements are generally performed by a dual-detector neutron porosity logging tool provided with a neutron emitting source that irradiates the formation under study. The tool is typically urged against one side of the borehole wall by tool eccentralizers. The resulting neutron population is sampled by at least two neutron detectors spaced at different distances from the source. A tool of this sort is described in detail in U.S. Pat. No. 3,483,376. If a two-detector measurement is made at a sufficient distance from the source, the effects of borehole size and tool standoff are minimized by taking the ratio of the counting rates. The ratio is, therefore, the measured parameter to compute porosity. Corrections are made to the porosity value computed from the ratio in order to improve accuracy. Although much smaller than for single detector systems, borehole diameter corrections for dual detectors systems are significant and can be quantified if the effective borehole diameter is known. Various borehole caliper devices were, and today still are, run in conjunction with dual detector neutron devices to provide a measure of borehole diameter from which a borehole size correction is computed and applied to porosity values computed from the ratio of detector responses.
Means for correcting dual detector neutron porosity devices, without using borehole diameter measurements from a caliper device, have been disclosed. U.S. Pat. No. 4,423,323 to Darwin V. Ellis and Charles Flaum, issued Dec. 27, 1983, describes a logging method for investigating the porosity of a sub-surface geological formation that includes the steps of:                passing a neutron logging tool through the borehole while irradiating the formation with neutrons;        detecting neutrons by a near and a far detector spaced from the source by different distances,        generating signals indicative of the near and far detectors count rates;        comparing the logarithms of the count rates to an empirically or mathematically derived tool response to variations only in porosity with the aid of another empirically or mathematically derived tool response to variations only in standoff and/or borehole size; and        generating a tangible representation of formation porosity corrected for the effects of standoff and/or borehole size from said comparison.        
This algorithm is however relatively complex, and the range of borehole diameter variation over which reliable compensation can be obtained is relatively limited.
Measurement-while-drilling (MWD) services were introduced commercially in the 1970's. These systems were typically mounted within drill collars and positioned as close to the drill bit as possible. Early MWD systems were directed toward the measurement of critical drilling parameters such as weight and the torque on the drill bit and direction of the drilled borehole. The operational and commercial value of such measurements is well known in the art. Subsequently, systems, which measured formation characteristics were introduced. Since such measurements provide information similar to wireline logging measurements, they are commonly referred to as logging-while-drilling (LWD) systems. There are many advantages in measuring formation parameters while drilling the borehole, rather than after the borehole has been drilled. The operational, financial, and technical advantages of LWD are likewise well known in the art. Neutron porosity, formation density, natural gamma ray, and various formation resistivity measurements were precursors to the present suite of available LWD measurements.
The earliest neutron porosity LWD systems employed only a single detector, but a second detector was quickly added. As in their wireline counterparts, the response of LWD dual detector neutron porosity systems is affected by borehole diameter and by the radial position of the source-detector system within the borehole. It is obvious that mechanical, arm type borehole calipers, which are used in wireline operations, cannot be used in LWD operations due to the rotation of the drill bit. Likewise, it is more difficult to control the radial positioning, or eccentricity, of the drill collar containing the LWD system within the borehole since wireline type mechanical centralizers or eccentralizers are not practical on a rotating drill string.
Various methods have been used to estimate the borehole diameter and drill string eccentricity in the immediate vicinity of the neutron porosity device. Estimates can be obtained from the drill bit diameter, the drilling fluid pumping pressure, and the mechanical properties of the formation being penetrated. Formation mechanical properties are estimated from MWD measurements, such as torque and weight on the bit, combined with rate of penetration of the drill bit that is measured at the surface. This method, at best, provides only a rough estimate of borehole geometry in the vicinity of the drill bit since formation and drilling mechanical conditions can change rapidly.
Other methods have been employed in an attempt to reliably caliper the borehole without using a specifically dedicated LWD caliper system. Generally speaking, these methods combine data from a plurality of LWD devices that exhibit different sensitivities to borehole geometric parameters. Such additional LWD devices might include well-known scattered gamma ray density devices and resistivity devices, which respond to varying radial depths of the borehole and formation environs. Borehole information is extracted by combining responses of these devices, and borehole corrections are derived from these responses. Again, generally speaking, this method of calipering a borehole and correcting measurements for borehole effects is not reliable. In addition, a relatively complex suite of LWD devices must be employed in order to practice this method.
U.S. Pat. No. 5,175,429 to Hugh E. Hall. Jr. et al, issued Dec. 29, 1992, discloses a tool stand-off compensation method for nuclear logging-while-drilling measurements. No independent borehole caliper or any other subsystem is required to obtain the desired tool stand-off or borehole size compensation. Count rates from a plurality of nuclear detectors are sorted and stored in “bins” as a function of apparent instrument stand-off. Detector responses are examined as a function of energy level thereby requiring spectral recording capabilities in the borehole instrument. These required features greatly increase the complexity of the borehole instrument, increase the demands on the logging-while-drilling telemetry system, and necessitate a relatively complex interpretation algorithm.
Document U.S. Pat. No. 5,767,510 describes means modifying the ratio of near detector to far detector count rates. A function of the far detector count rate has been found that results in a near detector response and a modified far detector response that exhibit nearly identical apparent radial sensitivities over the normal operating range of the tool. The result is a “modified” ratio of near detector count rate to modified far detector count rate that varies with formation, but that is essentially insensitive to radial perturbations such as variations in borehole diameter, variations in borehole shape, and variations in tool standoff.
An object of the present invention is to provide a borehole invariant neutron porosity measurement over a range of borehole diameters in which most dual detector neutron porosity devices are designed to operate. Another object is to provide a borehole invariant neutron porosity system that requires minimum data transmission and storage capacity. There are other objects and advantages of the present invention that will become apparent in the following disclosure.